1. Field of the Invention
The present invention relates generally to an active suspension system for an automotive vehicle. More specifically, the invention relates to an active suspension system which can perform fail-safe operation upon detection of failure of the active suspension system and can automatically resume normal state of suspension control when operation of the active suspension system resumes into normal state and a predetermined vehicular driving condition is satisfied.
2. Description of the Background Art
Japanese Patent First (unexamined) Publication (Tokkai) Heisei 1-145215 discloses an active suspension system with a fail-safe system. In the disclosure, the system has hydraulic cylinders at respective suspension systems disposed between a vehicular body and road wheels. Respective of the hydraulic cylinders have working chambers externally connected to a fluid pressure source to receive therefrom a line pressure. Pressure control valves are provided in a hydraulic circuit so that the fluid pressure in each of the working chambers of the hydraulic cylinders can be controlled independently of each other. The hydraulic circuit further includes an operational check valve disposed in a drain line and an one-way check valve in a supply line for preventing surge flow. The operational check valve and the one-way check valve forms pressure holding mechanism for maintaining line pressure in the vicinity of a predetermined neutral pressure which is set at a pressure level sufficient for maintaining a vehicular height at a neutral or target height level at a predetermined standard vehicular load of passengers and luggage. For enabling this, the operational check valve is designed to be operated between flow permitting position while the line pressure supplied to the pressure control valve is maintained higher than a predetermined pressure, e.g. in the vicinity of the neutral pressure, and flow blocking position while the line pressure to be supplied to the pressure control valve is held below the set pressure. The system is further provided with a fail-safe valve disposed in the hydraulic circuit. The fail-safe valve normally permits fluid flow through the supply line and the drain line for supplying line pressure for the pressure control valve and for draining pressurized fluid from the pressure control valve to a fluid pressure source through the drain line. The fail-safe valve is operated into a fail-safe mode position in response to detection of failure of active suspension system for blocking fluid flow both through the supply line and the drain line in order to establish a closed circuit across the pressure control valve and the working chambers for temporarily holding the fluid pressure in the working chambers at an instantaneous fluid pressure level and subsequently adjusting the fluid pressure in the working chambers toward the neutral pressure.
In such prior proposed system, although such prior proposed system is successful to back-up failure of the active suspension system by permitting vehicular travel by placing the system at fail-safe mode, in which the suspension systems operates as usual passive suspension, the normal mode of suspension control is resumed in response to termination of the abnormal state of the active suspension system. For example, assuming the failure is caused by malfunction in software in a control unit which can be cured by resetting the control unit, normal state of suspension control can be resumed after temporary fail-safe operation. This manner of resumption of the normal state of suspension control can degrade vehicular driveability at certain vehicle driving condition. Namely, when fail-safe mode operation is initiated, the fluid pressure is maintained at neutral pressure. At this condition, the hydraulic cylinder operates in a manner equivalent to the ordinary passive damper, i.e. normal shock absorber, for generating damping force only by effect of flow restrictive orifices. Therefore, vehicular attitude change can be caused according to magnitude of longitudinal or lateral acceleration exerted on the vehicular body. When resumption of normal active suspension control is taken place at the vehicular condition in which the vehicle is caused pitching or rolling, anti-pitching or anti-rolling action is instantly taken place to start adjustment of the fluid pressure for regulating the vehicular attitude. As a result, sudden change of the fluid pressure in the working chamber can be caused for causing shifting of gravity center. Since the pitching and rolling on the vehicular body is caused at relatively unstable condition of the vehicle, such sudden shifting of the gravity center requires additional control of vehicular behavior. This should not be pleasant or comfortable for the driver.